The objectives of this project are to assess changes in properties (number, affinity) of receptors in response to alterations in the environment, drug administration, stress, or hormones, and to develop methods for assessing receptor function in intact animals, in vivo. This year the sympatho-adrenal medullary and endocrine responses to hemorrhage have been examined in detail and the essential role of vasopressin in the recovery from hypotension discovered. Studies on the role of prostaglandins in regulation of blood pressure have continued implicated in the pathogenesis of the hypertension in experimental animals. Studies of response in intact, awake animals to a variety of stressors and stimuli provide a necessary complement to studies in pithed rats (project 201 MH 401). In the intact animal various reflex mechanisms and hormonal responses not possible in pithed rats can be recruited to most challenges to the homeostatic mechanisms, and the role of the central nervous system in organizing and regulating these responses can be examined.